U.S. Pat. No. 5,010,264 discloses a thermally protected electric motor such as this comprising a housing having an end cap at one end; a rotor, incorporating a commutator, mounted within the housing; brushes, engaging the commutator, and terminals, for connecting the brushes to an external electric supply, mounted within the end cap; a laminate PTC resistor mounted within the end cap and having two substantially parallel opposed surfaces; and two electrically conductive contact members connected, respectively, to one of the brushes and one of the terminals and respectively engaging the two surfaces.
Although this form of construction provides compact accommodation for the laminate PTC resistor, most PTC resistors have significant internal resistance which detracts from the viability of this form of protection. Moreover, with the prior art form of construction, it is difficult to overcome this deficiency by using a low resistance laminate PTC resistor in the form of a laminated, metal-polymer-metal PTC resistor such as that used in circuit protection devices sold by Raychem Corporation of Menlo Park, Calif., U.S.A., under the trademark "POLYSWITCH" and disclosed in U.S. Pat. Nos. 4,237,441, 4,238,812, 4,255,698 and 4,315,237.
This is because the operation of a laminated, metal-polymer-metal PTC resistor is critically impaired by excessive localized contact pressure, which deforms the layers and adversely affects the variable conductivity of the polymer layer, and by the conduction of heat between the PTC resistor and its surrounding. In the prior art construction, the two contact members are required to clamp the PTC resistor in place and so must be coextensive with substantial areas of the transverse surfaces of the PTC resistor. As these contact members are thermally conductive, they constitute either random heat sinks which absorb heat generated in the PTC resistor by current flow through the PTC resistor or random heat sources which transmit heat generated elsewhere to the PTC resistor. Clearly, where a laminated, metal-polymer-metal PTC resistor is used, this flow of heat into or out of the PTC resistor adversely affects the protective operation of the PTC resistor. However, if the contact areas are reduced, there is a danger that the contact pressure will become excessive.